The advantages of silicon carbide as a refractory corrosion-resistant material, both as a component of devices which are to be exposed to corrosive atmospheres or high temperatures and as a coating or surface layer for bodies which are to be subjected to thermal and chemical stress have long been recognized.
For example, in machines and apparatus which must be subjected to extremely high temperatures for which refractory metals are not suitable, silicon carbide elements may be employed or metal bodies may be coated with silicon carbide.
Silicon carbide has also been found to be applicable in many locations where its resistance to attack by chlorine, sulfur, oxygen and strong acids is important. Thus it is not uncommon to find silicon carbide bodies serving as chemical process apparatus or in chemical reactors and in machines such as turbines subjected to extremely high temperatures and attacked by corrosive gases at such elevated temperatures.
It has already been proposed to fabricate such bodies from silicon carbide by a hot-pressing method which utilizes the sinterability of silicon carbide powders under pressure.
For bodies of more complex shape, for example, the silicon carbide powder is placed in a die of appropriate shape and subjected to elevated pressures at a temperature between 1800.degree. C. and 2000.degree. C. to sinter the particles together and form a coherent article.
British Pat. No. 866,813 and U.S. Pat. No. 3,275,722, moreover, teach the admixture of binders to powders containing silicon carbide, the drying of the mixture, the pulverization of the resulting agglomerate, and the pressing of the powder thereby obtained to so-called green preforms. The latter can be thermally treated to eliminate the binder and the thermally treated bodies sintered at a temperature of about 2000.degree. C. in vacuum or the temperatures between 1500.degree. C. and 1700.degree. C. in a silicon vapor atmosphere which increases the silicon content of the fused body.
Experience with these methods has shown that they can be used only for articles of relatively simple shape if time-consuming machining and finishing steps are not desired. For more complex shapes, however, such machining steps are essential. Because of the high hardness of the silicon carbide bodies, the machining thereof is time-consuming and expensive requiring special tools and handling procedures.
Complex bodies, for example ceramic heat exchangers, parts of gas turbines, metal-smelting crucibles, generally must be assembled from a number of parts. When these bodies are composed of ceramics, the parts are bonded together by a ceramic-like composition, e.g. a slip which is applied between the prefabricated parts and after a firing of the assembled body, fuses with ceramic bond the parts together at their abutting surfaces. Such a process has been described in German Pat. No. 1,796,279.
Generally the parts which are joined by this method have previously been fired in accordance with ceramic handling technique so that the firing which joins the parts utilizing the ceramic slip is an after-firing.
This process has several drawbacks. For example, the quality of the slip generally differs from the quality of the parts which are to be joined thereby and frequently the resulting bond or Junction is not as strong as the parts which have been thus bonded. Furthermore, the difference in qualities between the slip and the ceramic of the parts Joined thereby may introduce stress differentials when the finished body is subjected to high temperatures or pressures.
It has also been proposed to join parts composed of graphite or graphite-like materials, hereinafter referred to collectively as carbonaceous materials, by the use of metal foils which are disposed at the junction. In this process, a foil is inserted between the abutting surfaces of the bodies to be Joined and the assembly is heated under pressure to bring about a reaction between the foil and the carbon of the graphite or graphite-like bodies. As a result, a film is formed at the junction which consis of a metal carbide of a high strength (see D. H. Sandstrom, Joining Graphite to Graphite with Transition Metal Foils, Los Alamos Scientific laboratory of the University of California LA--3960, Los Alamos, 1968.
However, when attempts are made to utilize the same approach with a thin layer of silicon, correspondingly effective results are not obtained. Furthermore, the fabrication of silicon in such foil thicknesses is extremely expensive and impractical.
British Pat. No. 713,710 describes a process for producing silicon carbide bodies or for coating graphite or graphite-like bodies with silicon carbide in which carbide layers are produced by vapor-depositing silicon in an inert atmosphere on the substrate or by immersing the graphite substrate in a silicon melt.
The immersion of bodies of graphite in metal melts to ultimately form protective layers of metal carbide on such bodies is also described in U.S. Pat. No. 2,929,714.
In both cases, however, complex structures in one piece can only be formed with machining, a disadvantage which has been illustrated with respect to the sintered bodies. Of course, the bodies can be assembled and joined by the use of ceramic slip into more complex structures, thereby avoiding the machining, but imbuin the process with the disadvantages which were discussed above in connection with the bonding of ceramic bodies with such slips.
From the foregoing, therefore, it will be apparent that prior to the present invention, there has been no fully successful method of fabricating silicon carbide bodies or body composed of graphite or graphite-like materials with silicon carbide coatings, of high complexity without the need for machining and in an economical manner such that the bodies are capable of withstanding thermal and mechanical stress and corrosive substances in an effective manner.